<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    var obj = {
      name: "why",
      age: 18,
    }

    var objJSONString = JSON.stringify(obj)
    console.log(objJSONString)

    var newObj = JSON.parse(objJSONString, function (key, value) {
      if (key === 'age') {
        return value + 2
      }
      return value
    })
    console.log(newObj)

  </script>
</body>

</html>